This invention relates generally to the field of non-destructive examination, and more specifically to the non-destructive examination of portions of a steam turbine apparatus, and particularly to an apparatus and method for the remote inspection of the inlet sleeve area of a high pressure steam turbine.
Steam turbines are well known in the power generation industry. A steam turbine is a device operable to extract heat energy from a flow of high pressure, high temperature steam and to convert that heat energy into mechanical energy in the form of the rotation of a shaft. The steam flow may be generated by any known type of steam generator, such as for example a fossil fueled boiler or a nuclear powered steam supply system. The rotating shaft of the turbine is commonly connected to a rotor shaft of an electrical generator to further convert the mechanical energy of the rotating shaft into electrical energy for distribution via the electrical power grid.
A typical steam turbine is illustrated in FIG. 1. The steam turbine 10 includes a rotor shaft 12 journaled for rotation within an inner cylinder 14 and an outer cylinder 16. The inner cylinder 14 includes, among other parts, a blade carrier ring 18 and several nozzle chamber units 20 each welded to the inner cylinder so as to become an integral part thereof. The outer cylinder 16 includes one or more high pressure steam inlets 22 and a number of inlet sleeve units 24, each of which extends inwardly in telescoping relation to its associated nozzle chamber 20 in the inner cylinder 14. Steam enters the turbine inlet 22 from a high pressure steam line (not shown) downstream from one or more control valves (not shown), into a nozzle chamber 20 integrally attached to the inner cylinder 14. The steam then passes through the nozzle and control stage rotating blades 26 that are attached to the rotor shaft 12. Steam from several parallel inlet paths flows into a control stage chamber 27 and around the various nozzle units 20 to merge together to flow through the remainder of the turbine array of stationary 28 and rotating 29 blade rows. The expanded steam exiting the last blade row enters a steam outlet annulus 36 formed between the inner and outer cylinders 14, 16 and is directed to an outlet 38.
The inlet steam flow must pass between the inner and outer cylinders 14, 16 without leakage between the cylinders. This requires a static seal that will withstand extremely high pressures, high temperatures, and differential thermal expansion. The seal must be substantially fluid tight and stable under conditions of extremely high velocity and sometimes pulsating steam flow. Dynamic instability, vibration and thermal shock are repeatedly encountered in use by the seal assembly. It is know to use a bell seal 30 for this application. Several known designs of such bell seals are described in U.S. Pat. No. 3,907,308 dated Sep. 23, 1975; U.S. Pat. No. 4,802,679 dated Feb. 7, 1989; and U.S. Pat. No. 4,812,105 dated Mar. 14, 1989.
Reliable operation of a steam turbine is desired in order to ensure the integrity of the electrical power supply and to avoid unplanned, and therefore more costly, repairs resulting from failures during the operation of the turbine. A variety of routine inspections are performed on a steam turbine to assess the condition of the machine during its useful operating life, and to detect degraded conditions before they mature into a part failure. The inlet sleeve area of a turbine is subject to extremes of temperature, thermal shock, vibration, and differential expansion, and as such, is an area vulnerable to mechanical wear and cracking. In particular, it is known that the surface 32 of the inner cylinder 14 in contact with the bell seal 30 is subject to wear. Such wear can result in a decrease in the effectiveness of the bell seal 30 and a greater leakage between the inner cylinder 14 and the outer cylinder 16 than desired. Furthermore, the trepan radius area 34 of the outer cylinder inlet sleeve 24 has been known to develop high cycle fatigue cracks in some turbines. It is known to inspect portions of a steam turbine by inserting a miniature camera into the turbine through the main steam inlet nozzle 22, such as is taught by U.S. Pat. No. 5,164,826 dated Nov. 17, 1992. However, inspections of the bell seal and trepan radius areas 30, 34 have previously been performed with the turbine out of service and with the turbine casing disassembled to provide access to these parts. Consequently, these inspections are time consuming and expensive.
Once the turbine is disassembled, the bell seal 30 may be visually inspected and measured for wear. The trepan radius area 34 is, however, too restricted to permit a reliable visual inspection. It is known to inspect this area with a special magnetic rubber material. The trepan radius area 34 must first be cleaned of accumulated scale and dirt such as by grit blasting. Special bladders are inserted into the trepan groove 35 to provide a sealed cavity therein. A multi-loop coil is wrapped around the outside of the inlet sleeve 24, and a liquid magnetic rubber material is then pumped into the sealed cavity. An electrical current is passed through the multi-loop coil to produce a magnetic field within the inlet sleeve 24. Cracks in the trepan area 34 will act as flux leakage sites and will draw small magnetic particles in the liquid magnetic rubber material toward the flux leakage sites. As the liquid rubber solidifies, this build up of magnetic particles is captured and can be interpreted as an indication of cracks in the trepan area 34 by a skilled non-destructive examination technician. This type of inspection is generally performed only during scheduled turbine maintenance outages when the turbine is being disassembled for other purposes, and the information provided about flaws in the trepan radius area is affected by the inherent limitations of electromagnetic testing techniques.
Thus there is a particular need for an inspection technique that provides an improved non-destructive examination of the turbine inlet sleeve area without the need for the disassembly of the turbine.
Accordingly, a method of inspecting the inlet sleeve area of a steam turbine is described herein, the method comprising the steps of: providing an inspection tool adapted for insertion into a steam inlet of the steam turbine, the inspection tool including a spaced pair of inflatable bladders and an ultrasonic transducer disposed there between; inserting the inspection tool into the steam inlet and moving it into an inspection position proximate the inlet sleeve; pressurizing the pair of inflatable bladders to form a sealed area surrounding the ultrasonic transducer; introducing liquid couplant into the sealed area; operating the ultrasonic transducer to perform a non-destructive examination of the inlet sleeve area; depressurizing the pair of inflatable bladders; and withdrawing the inspection tool from the steam inlet. The method may further include the steps of: providing a source of light and a camera on the inspection tool; and monitoring the output of the camera during the step of inserting the inspection tool to identify the inspection position when light produced by the source of light impinges upon a predetermined structure proximate the inspection tool.
An apparatus for implementing the disclosed method of inspecting the inlet sleeve of a turbine is also described. The apparatus includes a guide tube adapted for insertion into a steam line connected to a turbine; an ultrasonic transducer movably connected about an inspection section of the guide tube for remote operation of the transducer; an actuator connected between the guide tube and the ultrasonic transducer for selectively and remotely moving the transducer relative to the inspection section for inspecting a surrounding structure; a leading inflatable bladder and a trailing inflatable bladder each attached about the guide tube on opposed sides of the inspection section; and a couplant supply line having an opening between the leading and trailing inflatable bladders for selectively and remotely providing couplant to a volume between the leading and trailing bladders including the ultrasonic transducer. The apparatus may also include an optical positioning device attached to the guide tube for providing a remote indication of the position of the inspection section. The optical positioning device may be a laser for projecting a beam of light; and a camera for remotely monitoring the location of impingement of the beam of light.
A tool for providing both axial and rotational movement of the ultrasonic transducer in the inspection apparatus may include a shaft having an axis; a pattern of spur gear teeth formed on a first portion of the surface of the shaft, the pattern of spur gear teeth formed in a circumferential direction about the surface of the shaft; a pattern of rack gear teeth formed on the first portion of the surface of the shaft, the pattern of rack gear teeth formed in a longitudinal direction about the surface of the shaft; a driven oscillator gear engaged with the pattern of spur gear teeth for imparting rotation of the shaft about the axis; and a driven axial spur gear engaged with the pattern of rack gear teeth for imparting axial movement of the shaft along the axis.